1. Field of the Invention
The present invention relates to a video display apparatus, and more particularly, to a surge protector for the video display apparatus capable of preventing the video display apparatus from malfunctioning due to a drastic change of high voltage from a discharge of a cathode-ray tube or external circumstances.
2. Description of the Prior Art
In general, in a video display apparatus which adopts the cathode-ray tube using a high voltage, such as a monitor of a computer system, a surge voltage is often generated due to a display mode change, a power source on/off or a display power management system mode change operation.
An earlier high voltage generating circuit of the video display apparatus has a microcomputer for receiving horizontal and vertical sync signals H-SYNC and V-SYNC (hereinafter, referred to as H/V sync signals) and a plurality of control signals, a horizontal/vertical processor (hereinafter, referred to as an H/V processor) for generating horizontal and vertical oscillation signals in response to H/V sync signals H-SYNC and V-SYNC from the microcomputer to output a switching signal, a switching circuit for outputting an external voltage Vcc in response to a switching signal, a high voltage generator for amplifying the voltage Vcc from the switching circuit to supply a high voltage to a cathode-ray tube CRT, and a constant-voltage circuit for comparing a voltage signal from the high voltage generator with a predetermined reference voltage and for outputting a constant-voltage control signal to control a duty cycle of the voltage Vcc supplied to the high voltage generator.
If the voltage signal is higher than the predetermined reference voltage, the output voltage Vcc which is supplied to the high voltage generator goes down because the duty cycle of the voltage Vcc decreases due to the constant-voltage control signal.
On the contrary, if the voltage signal is lower than the predetermined reference voltage, the output voltage Vcc which is supplied to high voltage generator goes up because the duty cycle of the voltage Vcc increases due to the constant-voltage control signal. Accordingly, the high voltage generated by high voltage generator usually maintains a uniform voltage level.
However, if the high voltage supplied to the cathode-ray tube is drastically reduced by impurities inside the cathode-ray tube or other factors, the voltage level of the high voltage from the high voltage generator lowers. As a result, since the voltage level of the voltage signal inputted to the constant-voltage circuit lowers, the duty cycle of the voltage Vcc increases due to the constant-voltage control signal from the constant-voltage circuit.
According, an overcurrent flows from the switching circuit to the high voltage generator and the high voltage generator malfunctions due to the overcurrent.